Original Wabco ABS on my 89 RR is frequently failing to switch the brake pump on with attendant brake warning light coming on etc. Knocking the front most relay (warning light relay) gets it going. Yep replaced this relay twice but still same. Relay also seems to get hot or is this normal. Can't figure from the wiring diagram how tripping the warning light relay gets the pump going or is it all due to a fault somewhere else like the pressure switch on the brake pump? Any ideas appreciated.